In general, glasses as industrial products are produced by heating raw material powders prepared in a prescribed mixing proportion in a crucible or a tank furnace at temperatures higher than the liquids temperature to form a homogeneous mixture in the melted state and then cooling the mixture. In the production of a glass, the glass is usually made transparent, for example, by a means in which the bubbles in the melt formed from the adsorbed gas in the raw materials and the gas generated during the reaction are removed by elevating the temperature of the melt to reduce its viscosity sufficiently thereby allowing the gases and bubbles to float to its surface.
However, in the case of producing a glass from silica as the raw material, because of its high melting point, the temperature cannot be elevated to an extent effective for removing bubbles of restrictions such as the high viscosity of the melt, refractoriness required for the crucible or furnace and for other reasons. If the temperature is elevated excessively, gases are generated by the volatilization of the raw material per se and the reaction between the raw material and the crucible forms all the more bubbles. Therefore, the above-described method cannot be employed. For the reasons set forth above, a method for the production of a transparent quartz glass using silica as the raw material is restricted to any one of the following methods:
(1) A method in which a silica powder is gradually fed into an argon-oxygen plasma flame or an oxygen-hydrogen flame and melted for glass formation and the resulting melt is deposited onto a stand. The generated gases are dissipated from the surface (Verneuil method);
(2) A method in which a porous body composed of silica fine particles is prepared and melted from one end thereof in the band-like state for glass formation. The generated gases leave from the unmelted porous body (zone melting method); and
(3) A method in which rock crystal powders prepared to have particle diameter of about 100 .mu.m are placed in a crucible and melted in a vacuum furnace for glass formation. The generated gases are removed by force (vacuum melting method).
However, with respect to the method (1) and method (2), it is well known that an extremely long period of time is required for producing one glass block and productivity is poor, and especially in the case of the Verneuil method, the yield is as low as 30% to 40%. Further, in the case that the argon-oxygen plasma flame is employed as a heat source, though a glass having a small number of residual OH groups and a relatively small number of bubbles can be obtained, the energy cost is high, whereas in the case that the oxygen-hydrogen flame which is low in the energy cost is employed, the problem is that the product has a large number of residual-OH groups. Still further, since the shape of ingots which can be produced is restricted to cylindrical and slender shapes there is a disadvantage to the subsequent processings.
According to the vacuum melting method (3), though a relatively large-sized ingot having a small number of residual OH groups and a high viscosity at high temperatures can be obtained, since the raw material powder filled in a vessel such as a crucible is melted for glass formation, not only is there a difficulty in degassing but also a reaction gas caused by the contact with the vessel is generated, and the resulting glass has a relatively large number of bubbles. Therefore, glass having a high quality cannot be obtained. Further, since the rock crystal powders are used; bubbles tend to be generated due to low purity of the raw material per se also, there is a concern in the raw material supply due to exhaustion of the resources.
Under such a background, the present inventors have developed a process by which a transparent or functional, high-quality glass can be readily produced at a low production cost. This process is characterized in that a silica powder is filled in a suitable vessel, heated in the presence of an accelerator for phase conversion such as an alkali metal component etc. to form a porous body having continuous open pores unified in a crystal phase right below the melting point, and then melted in vacuo for glass formation (see Japanese Patent Application Nos. 181586/84, 181587/84, 181588/84, 170663/85, and 170664/85).
It is known that crystalline silica causes phase conversion from a quartz phase in low-temperature to a tridymite phase and further to a cristobalite phase, depending upon the heating temperature in the heating step. This phase conversion hardly takes place when silica is used alone, but Li.sub.2 O, Na.sub.2 O, K.sub.2 O, MgO, CaO, P.sub.2 O.sub.5, and B.sub.2 O.sub.3 are known to be useful as an accelerator for phase conversion. Since amorphous silica is sintered for glass formation without being crystallized when used alone, in order to crystalize it into a cristobalite phase, the addition of the above-described metal additives is required. Accordingly, in the process of the present invention, the above-described metal component is used in the step for crystallizing the silica powder into the cristobalite phase. However, as will be understood from the foregoing explanation concerning the prior art technologies, in the conventional production methods of glasses, if such a metal component is present in the raw material, such metal component, if any, is likely to cause deterionation in purity of a final product as in the case of OH groups or the like and this is not desirable. That is, in the conventional production methods of glasses, there is a conflicting relation between obtaining a high-quality quartz glass and adding impurities to the raw material or employing a raw material containing impurities.
In view of the foregoing fact, it can be said that the process of the present invention in which an accelerator for phase conversion is added to silica or silica containing an effective component for the phase conversion is selectively used as the raw material is a method not available with the conventional concept. The reasons why this process can readily obtain a high-quality glass as compared with the conventional methods is that the characteristics available with a sintered body consisting of a cristobalite phase produce improved effects when coupled with employment of the vacuum melting method. In other words, as is well known, since the melting point of the sintered body is uniquely determined by the cristobalite phase, the sintered body can be heated up to the temperature right below the melting point and subjected to the degassing processing. Further, since the sintered body consisting of a cristobalite phase is a porous body having continuous open pores, it can be degassed thoroughly and readily. Accordingly, if a metal component, such as Na, which is readily decomposed and vaporized at temperatures below the melting point of the sintered body, is employed as an accelerator for phase conversion, a transparent quartz glass from which the impurities (including the accelerator for phase conversion) have almost completely been removed can be obtained. On the other hand, if an accelerator which is not decomposed and removed at said melting point is selected, a functional glass containing only the accelerator but free from other impurites can be obtained.
The process for production of a glass with which the present invention is concerned provides high-quality glass ingots with good productivity through an organic combination of a crystallization process in which an accelerator for phase conversion is used with a glass forming process by the vacuum melting method, but also involves a drawback in obtaining glass ingots having a desired shape. This is because in the case where a glass ingot having a desired shape such as a plate-like shape or a cylindrical shape is intended, it is difficult to produce a satisfactory sintered body having the desired shape with good productivity.
Furthermore, though the process for production of a glass with which the present invention is concerned has an effect that a relatively large-sized glass ingot can be obtained, in order to obtain a larger ingot, it is necessary to enlarge the size of a container for the sintered body, and the manufacture of such a container becomes difficult, owing to the heat resistibility and other properties required for such a container. Much more, since silica powders to be used as the raw material are low in both bulk density and heat conductivity, in the case that a large-sized sintered body is intended, problems may occur either in production efficiency or in the quality of the ingot.
In view of the foregoings, an object of the present invention is, therefore, to provide a process for production of a glass comprising a crystallization process and a vacuum melting process, whereby a high-quality glass ingot having a desired shape or a larger glass ingot can be readily produced without reduction in productivity.